1. Field of the Invention
The present invention relates in general to a method of manufacturing a grindstone or grinding wheel including a cylindrical core body and a plurality of abrasive segment chips which are fixed to an outer circumferential surface of the cylindrical core body.
2. Discussion of the Related Art
There is known a segment-chip-type grinding wheel including: a cylindrical core body; and a plurality of abrasive segment chips which have respective abrasive layers and which are fixed to an outer circumferential surface of the cylindrical core body. In a grinding operation with this grinding wheel, the grinding wheel is rotated about an axis of the cylindrical core body, so that a workpiece is ground by the abrasive layer of each abrasive segment chip. The abrasive layer has a longer service life where the abrasive layer is formed of so-called “super abrasive grains” such as diamond abrasive grains and CBN (cubic boron nitrides) abrasive grains, than where the abrasive layer is formed of standard abrasive grains such as alumina abrasive grains and silicone carbide abrasive grains. Where the abrasive layer is formed of the super abrasive grains, the abrasive layer has a relatively small thickness, in general, due to a relative expensiveness of the super abrasive grains. The segment-chip-type grinding wheel having such a construction is widely used in various fields, while being studied for the purpose of further increasing its grinding performance. One example of such a segment-chip-type grinding wheel is described in the specification of the Japanese Patent Application 2001-053927 (corresponding to the U.S. patent application Ser. No. 10/080,686 and the German Patent Application No. 102 08 423.8), in which the abrasive segment chips are arranged in such a manner that effectively prevents a chattering or self-induced vibration in the grinding operation.
FIG. 1 is a perspective view showing a grinding wheel 10, as one example of the conventional segment-chip-type grinding wheel, in which a multiplicity of part-cylindrical or arcuate abrasive segment chips 12 are fixed to an outer circumferential surface of a cylindrical core body 14. As is apparent from FIG. 1, a plurality of the abrasive segment chips 12 are arranged as viewed in the circumferential direction of the cylindrical core body 14 and also as viewed in the axial direction of the cylindrical core body 14. FIG. 2 is a perspective view showing a grinding wheel 20, as another example of the conventional segment-chip-type grinding wheel, in which the arcuate abrasive segment chips 12 are fixed to the outer circumferential surface of the cylindrical core body 14. The grinding wheel 20 is different from the grinding wheel 10 in that the abrasive segment chips 12 include relatively long segment chips and relatively short segment chips which are alternately arranged.
The conventional segment-chip-type grinding wheels such as the above-described grinding wheels 10, 20 are widely used in a centerless grinding operation in which a cylindrical workpiece is not supported on its centers but rather by a work rest blade, a regulating wheel, and the grinding wheel. FIG. 3 is a view illustrating a thru-feed centerless grinding operation in which an outer circumferential surface of a cylindrical workpiece 22 is grounded by the grinding wheel 10. The workpiece 22, which is disposed on a work rest blade 16 and is guided by a work rest guides 18, is continuously fed in a longitudinal direction as indicated by the arrow, while being gripped by and between the grinding wheel 10 and the regulating wheel 24. The regulating wheel 24 is rotated for rotating workpiece 22 at a relatively low speed, while the grinding wheel 10 is rotated at a relatively high speed, whereby the outer circumferential surface of the workpiece 22 is grounded by the grinding wheel 10.
FIG. 4 is a view illustrating an infeed centerless grinding operation in which a cylindrical workpiece 26 having a shoulder is grounded by the grinding wheels 20 and a grinding wheel 32 which has a diameter smaller than that of the grinding wheel 20. A small diameter portion of the workpiece 26 is gripped by and between the grinding wheel 20 and a regulating wheel 30, while a large diameter portion of the workpiece 26 is gripped by and between the grinding wheel 32 and a regulating wheel 34 which has a diameter smaller than that of the regulating wheel 30. The regulating wheels 30, 34 are rotated for rotating workpiece 26 at a relatively low speed, while the grinding wheels 20, 32 are rotated at a relatively high speed and are fed in a transversal direction toward and away from the workpiece 26 as indicated by the arrows, whereby the outer circumferential surface of the workpiece 26 is grounded by the grinding wheel 10. It is noted that reference numeral 28 denotes a stopper which is provided to position the workpiece 26 in a predetermined longitudinal position relative to the grinding wheels 20, 32.
However, there have been discussed problems which could be caused in a centerless grinding operation with the conventional segment-chip-type grinding wheel 10 or 20 in which a plurality of the abrasive segment chips 12 are arranged as viewed in the axial direction of the cylindrical core body 14, namely, in a direction perpendicular to an end face of the cylindrical core body 14. For example, in the thru-feed centerless grinding operation shown in FIG. 3, there is a possibility that the workpiece 22 could jump upon its contact with a joint clearance or joint line between the adjacent abrasive segment chips 12 while being fed across the grinding wheel 10 and the regulating wheel 24. That is, there is a risk that the grinding wheel 10 could be damaged by the jumping workpiece. Further, in the infeed centerless grinding operation shown in FIG. 4, there is a possibility that the ground workpiece 26 could suffer from a low degree of roundness and undesirable marks generated on the ground surface, if there is a gap or clearance between the adjacent abrasive segment chips 12, or if there is an adhesive (which was used for bonding the abrasive segment chips 12 to the cylindrical core body 14) sticking to a grinding surface which is constituted by surfaces of the abrasive segment chips 12. Although there have been practiced various manners for preventing the adhesive from being exposed on the grinding surface, it is extremely difficult to completely eliminate the adhesive exposed on the grinding wheel. Particularly, in the segment-chip-type grinding wheel 10 or 20 in which the joint lines extend not only in the axial direction but also in the circumferential direction, it is practically impossible to eliminate the adhesive exposed on the grinding wheel.
The above-described problems might be solved by employing a segment-chip-type grinding wheel 50, as shown in FIG. 9, which has joint lines extending in the axial direction but does not have any joint line extending in the circumferential direction, namely, which has part-cylindrical abrasive segment chips 52 each extending over the entire axial length of the cylindrical core body 14.
However, this segment-chip-type grinding wheel 50 is difficult to be produced by conventional techniques, due to a considerably large length of each abrasive segment chip 52. That is, there is no conventional technique for satisfactorily firing unfired precursor to prepare such an abrasive segment chip 52 having the considerably large length, as shown in FIG. 5.
FIGS. 6 and 7 show conventional arrangements for firing unfired part-cylindrical precursor 54 to prepare the abrasive segment chip 52. In FIG. 6, the unfired part-cylindrical precursor 54 is disposed or stood on a setter 56 consisting of a flat plate such that the part-cylindrical precursor 54 is held in contact at one of lengthwise opposite end faces with a flat surface of the setter 56. In FIG. 7, the unfired part-cylindrical precursor 54 is disposed or stood on the setter 56 such that the part-cylindrical precursor 54 is held in contact at one of widthwise opposite end faces with the flat surface of the setter 56. However, since the precursor 54 is inevitably softened in the firing process, the precursor 54 is likely to be deformed due to its own weight in either one of the arrangements of FIGS. 6 and 7. In the arrangement of FIG. 6, the precursor 54 tends to be bent in the longitudinal direction. In the arrangement of FIG. 7, the precursor 54 tends to be bent in the width direction. Such a deformation of the precursor 54 makes it impossible to satisfactorily bond the precursor 54 to the cylindrical core body 14.
FIG. 8 shows another arrangement for firing the unfired part-cylindrical precursor 54. In this arrangement, the above-described setter 56 consisting of the flat plate is replaced with a setter 58 having a part-cylindrical surface 58t whose radius of curvature is substantially equal to that of a radially inner surface of the part-cylindrical precursor 54, so that precursor 54 can be disposed or laid on the setter 58 such that the part-cylindrical precursor 54 is held in close contact at the radially inner surface with the part-cylindrical surface 58t. Although this arrangement is effective to prevent the deformation of the precursor 54, the precursor 54 is likely to adhere to the setter 58 due to the close contact.
FIG. 11 shows a conventional arrangement for bonding the part-cylindrical abrasive segment chip 52 to the cylindrical core body 14. In this arrangement, the core body 14, the abrasive segment chip 52 and a magnet block 62 are disposed on a base plate 60 made of a metal such as a steel. The magnet block 62 is moved to force the abrasive segment chip 52 against the outer circumferential surface of the core body 14, such that the abrasive segment chip 52 is held in contact with the outer circumferential surface of the core body 14 via an adhesive. That is, the contact of the abrasive segment chip 52 with the core body 14 via the adhesive is held by the magnet block 62 which is fixed to the base plate 60 owing to a magnetic force, until the hardening of the adhesive is completed. This arrangement suffers from a problem that the abrasive segment chip 52 can not be satisfactorily bonded to the core body 14, even where the segment chip 52 is distorted or bent by a small amount.
As discussed above, there does not exist a satisfactory method of manufacturing a segment-chip-type grinding wheel with abrasive segment chips each having a large axial length, namely, a segment-chip-type grinding wheel with segment chips each extending over an entire axial length of the grinding wheel, although there is a demand for such a manufacturing method.